selftests: forwarding: Add a fallback cleanup()

Consistent use of defers obviates the need for a separate test-specific
cleanup function -- everything is just taken care of in defers. So in this
patch, introduce a cleanup() helper in the forwarding lib.sh, which calls
just pre_cleanup() and defer_scopes_cleanup(). Selftests are obviously
still free to override the function.

Since pre_cleanup() is too entangled with forwarding-specific minutia, the
function cannot currently be in net/lib.sh.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>